The SIP (Session Initiation Protocol) standard is defined as one of the communication control protocols for realizing call control such as connection request in a real-time application such as Internet phone that applies VoIP (Voice over Internet Protocol) (see Non-Patent Document 1).
The SIP at the beginning had been used for VoIP communications between computers such as PCs and servers that are connected through wired networks of the Internet. However, in these years, transition to digital and IP technologies has accelerated also with respect to the wireless communication apparatus, and SIP servers and wireless communication terminals of wireless communication carriers (operators) who support the SIP have been developed. Further, recently, with development of the wireless communication technology, one wireless communication terminal is able to have a plurality of wireless communication devices that can be connected to different wireless communication networks. A typical configuration is a PDA, a PC or a cellular phone having the first wireless communication device connected to a cellular phone network and the second wireless communication device connected to a station of wireless LAN and MAN (Metropolitan Area Network) such as WiFi and WiMAX.    Non-Patent Document 1: Website: http://www.ietf.org/rfc/rfc3261.txt (RFC document that defines the SIP).